Solid oxide electrochemical devices have demonstrated great potential for future power generation with high efficiency and low emission. Such solid oxide electrochemical devices include solid oxide fuel cells (SOFCs) and solid oxide electrolyzers.
Solid oxide fuel cells produce electricity by oxidizing fuel on one electrode (anode) and reducing oxygen on the other electrode (cathode). The electrodes are separated by an electrolyte that conducts electricity by the migration of ions. Under the appropriate conditions the reduction/oxidation reactions on the electrodes produce a voltage, which can then be used to generate a flow of direct current. In the case of a solid oxide fuel cell operating with hydrogen fuel and air as an oxidant, oxygen ions are conducted through the electrolyte where they combine with the hydrogen to form water as an exhaust product. The electrolyte is otherwise impermeable to both fuel and oxidant and merely conducts oxygen ions. This series of electrochemical reactions is the sole means of generating electric power within the solid oxide fuel cells. It is therefore desirable to reduce or eliminate any reactant mixing that results in a different reaction, such as combustion, which does not produce electric power and therefore reduces the efficiency of the solid oxide fuel cell.
Solid oxide electrochemical devices are typically assembled in electrical series in a solid oxide electrochemical device stack to produce power in useful amounts. To create a solid oxide electrochemical device stack, an interconnecting member, referred to as an interconnect, is used to connect the adjacent solid oxide electrochemical devices together in electrical series. Typically, an anode layer is connected to an anode interconnect and a cathode layer is connected to a cathode interconnect. When the solid oxide electrochemical devices are operated at high temperatures, such as between approximately 600° C. and 1000° C., the solid oxide electrochemical devices are subjected to mechanical and thermal loads that may create strain and resulting stress in the solid oxide electrochemical device stack.
Typically, high temperature solid oxide electrochemical devices are made of ceramics, which must be sealed to the metallic interconnect structure in order to define closed passages for the reactants, namely the fuel and the oxidant, to flow to and from the solid oxide electrochemical device. During thermal cycles, various components of the solid oxide electrochemical device stack expand and/or contract in different ways due to the difference in the coefficient of thermal expansion of the materials of construction. In addition, individual components may undergo expansion or contraction due to other phenomena, such as a change in the chemical state of one or more components. This difference in dimensional expansion and/or contraction may affect the seal separating the oxidant and the fuel paths and also the sealing of the elements made of dissimilar materials.
Conventionally, a typical anode layer of a solid oxide electrochemical device is made of a nickel based cermet, which itself is made by chemical reduction of nickel oxide in mixture with a ceramic. A major problem in solid oxide electrochemical device stack design is that the high temperature typically requires that the seals be made of brittle materials such as glass and glass ceramics. Prior to operation, the nickel oxide in the anode of the solid oxide electrochemical device is reduced to nickel at high temperature, and this chemical reduction causes a physical reduction of volume of the anode. This reduction in the volume of the anode layer can place additional stress on links between the solid oxide electrochemical device and other components, such as the seal, and can cause the seal of the solid oxide electrochemical device assembly or the solid oxide electrochemical device itself to fail. This stress is aggravated by the stresses arising from different coefficients of thermal expansion of the ceramic and metal, thereby causing the unequal physical reduction of volume of the anode layer and the interconnect in contact with the anode layer. Another consequence of the differential thermal and chemical expansions of the solid oxide electrochemical device and the interconnect is the potential loss of mechanical contact between the anode layer or cathode layer and its corresponding interconnect (the anode interconnect or the cathode interconnect).
In addition, conventional processing of multiple solid oxide electrochemical assemblies in a solid oxide electrochemical stack has relied upon sealing all or several of the solid oxide electrochemical assemblies and interconnects in a single process to form an integral, inseparable stack. If, following such assembly and processing, a defect is identified in any seal, the solid oxide electrochemical device stack cannot be disassembled without destroying the seals. Thus, any defect in one solid oxide electrochemical device assembly could render the entire solid oxide electrochemical device stack unusable.
A common approach to the thermal stress problem is to find a combination of ceramic and metal where the coefficients of thermal expansion match closely enough that stresses are minimized. However, it is very difficult to match the coefficients over the entire temperature range. Moreover, even such matching does not avoid stresses due to the reduction in volume of the anode layer in its pre-operation transition from a ceramic and nickel oxide mixture to a nickel based cermet. Also, the materials chosen based upon a close thermal match may not be optimal for the performance of the solid oxide electrochemical device.
Accordingly, there is a need for a simple and economically desirable design of a solid oxide electrochemical device stack and a method for making a solid oxide electrochemical device that avoids the above-described stresses and defects.